Many vehicles, particularly trucks having large hauling capacity, employ manual transmissions in their drivetrains. Such transmissions allow the vehicle operator to manually select one of several available gear ratios for use in transitioning between the speed of the engine and the vehicle road speed. For any given vehicle road speed, the selected gear ratio determines the engine speed. For example, suppose a vehicle having a road speed of 65 miles per hour (mph) may be operated in either ninth or tenth gear (the convention used throughout the present application is that a "lower" gear has a numerically higher gear ratio than a "higher" gear, hence a transmission's "first gear" will have the numerically highest gear ratio of all of the gears in the transmission). In the case, the engine speed when the transmission is in ninth gear will be greater than the engine speed when the transmission is in tenth gear, for any given vehicle road speed.
The selection of the engine speed for any given vehicle road speed has several important consequences. First, optimum fuel economy results from achieving highway speeds at relatively low engine speeds. Therefore, the highest possible gear should be selected at any given vehicle road speed if the desired result is maximum fuel economy. This is because the higher gear will result in a lower engine speed, and therefore better fuel economy.
Second, vehicle performance is also directly related to engine speed. Generally, the horsepower output of an internal combustion engine will increase with increasing engine speed (up to a relatively very high engine speed, at which point it will begin to decrease with increasing engine speed). Therefore, the lowest possible gear should be selected (while still maintaining the engine speed within its normal operating speed range) at any given vehicle road speed if the desired result is maximum performance.
It will be appreciated by those skilled in the art that maximum fuel economy and maximum engine performance are mutually exclusive goals. This presents a problem in the situation where the owner of a truck desires that the truck be operated in such a way as to maximize fuel economy, while the driver of the truck (the owner's employee) desires to operate the truck so as to maximize performance. Because such owners have no physical control over their drivers while they are operating their trucks, owners desire that the trucks be designed so as to automatically limit vehicle speed independent of the driver's wishes in an effort to improve fuel economy.
Prior art solutions to this problem utilize a control system which imposes a maximum vehicle speed limit for gears lower than top gear (i.e. gears with numerically higher ratios than top gear). Thus, if the vehicle is not in top gear( gear down operation), the control system will not allow the vehicle speed to increase beyond the limit for that gear. These prior art solutions do not allow proper operation of the vehicle during periods of legitimately high vehicle speed when not in top gear, such as when downshifting on uphill grades or when accelerating with the goal of upshifting.
There is therefore a need in the prior art for a control system which will limit the driver's ability to operate the vehicle at high speed when not in the top gear, but which will also allow operation of the vehicle in legitimate high speed/non-top gear driving maneuvers. The present invention is directed toward meeting this need.